The present system relates to a device for developing a charge image on a photoconductor of an electrophotographic printer or copier. A developer chamber has a first end and a second end. In the chamber at least a portion of the developer is contained. A mixing unit is provided for thoroughly mixing the developer in the developer chamber. The system further relates to a method for developing a charge image on a photoconductor of a printer or copier.
During the development of a charge image on a photoconductor, the developer is either applied to the charged areas of the photoconductor (in the so-called charged area development) or to the discharged areas of the photoconductor (in the so-called discharged area development). For example, a mixture of toner particles and magnetic carrier particles is used as a developer. During the thorough mixing of the developer in the developer chamber, the toner particles and the carrier particles are tribo-electrically charged by means of friction, as a result whereof the toner is charged as required for its application to the charge image. The magnetic carrier particles can be applied to the photoconductor with the aid of magnetic rollers, whereupon the toner particles adhering to the carrier particles are transferred from the carrier particles onto the charge image of the photoconductor. However, the system is not restricted to such developer mixtures but can, for example, also be used for one-component developers.
The mixing unit of the device is a multiple function mixing unit. On the one hand, it serves to thoroughly mix the developer in the developer chamber such that there results an almost uniform developer filling level over the entire width of the developer chamber. Width refers to the dimension of the developer chamber that is transverse to the direction of motion of the photoconductor relative to the developing device. It is important to have an at least almost uniform developer filing level over the width of the developer chamber to guarantee that the entire width of a roller or of several rollers applying the developer to the photoconductor is brought into contact with the developer so that the charge image on the photoconductor is developed completely and uniformly.
In case the developer comprises of a mixture of toner and carrier particles, toner particles have to be supplied to the mixture by the same amount as taken from the developer mixture during development of the charge image. This amount of toner supplied has to be mixed in evenly by the mixing unit, since an inhomogeneous mixture of toner and carrier particles would result in an inhomogeneous optical density of the print image. Finally, the mixing unit has to thoroughly mix the developer mixture such that the developer is activated, i.e. that the toner particles are sufficiently charged.
An at least almost uniform developer filling level over the width of the developer chamber is likewise significant with respect to a uniform charging of the developer mixture, since a varying filling level results in a varying mixing behavior and consequently in a non-uniform charging of the developer mixture.
In known developing devices, the mixing unit is formed by a so-called paddle wheel which is arranged in the developer chamber and the axis of which runs in the transverse direction of the developer chamber, i.e. transverse to the direction of motion of the photoconductor relative to the developer chamber. The paddle wheel has paddle-like or shovel-like blades, by means of which the developer is thoroughly mixed upon rotation of the paddle wheel.
While it is possible to efficiently circulate the developer by means of such a paddle wheel, it is relatively difficult to thoroughly mix the developer in the transverse direction, i.e. along the axis of the paddle wheel, in an efficient way. The difficulty in thoroughly mixing in the transverse direction is that, in spite of the thorough mixing in the transverse direction, the filling level has to remain at least almost the same over the width of the developer chamber. As a result thereof, the developer has to be thoroughly mixed in the transverse direction such that there is no net flow of developer in the transverse direction.
The prior art discloses paddle wheels having radially outer blades, which generate a toner flow in a transverse direction, and having radially inner blades, which generate a toner flow in the opposite transverse direction. These radially inner and radially outer blades are designed such that the developer mixture is transported in both transverse directions at the same transport rate so that no net flow of developer mixture in the transverse direction occurs and thus the filling level of the developer in the developer chamber remains at least almost constant in time over the width of the developer chamber.
In order to achieve an efficient thorough mixing in the transverse direction without or with a low net flow in the transverse direction, both the revolutions per minute of the paddle wheel as well as the flow properties of the developer must be very close to a desired value for which the paddle wheel is designed. If the revolutions per minute of the paddle wheel or the flow properties of the developer even only moderately deviate from the desired value, in conventional developing units already a net flow of the developer in the developer chamber occurs and results in an accumulation of developer on one side only, and therefore in deterioration of the developing quality.
Since the flow properties of a developer mixture of toner and carrier particles vary with the toner concentration, in known devices the toner concentration in the mixture has to be kept very close to a desired value at relatively great expense in order to not deteriorate the thorough mixing in the transverse direction. This not only requires great expense but also prohibits the control of the optical density of the print image by means of the toner concentration, which is a significant restriction. In addition, the flow properties of the developer mixture are also dependent on climatic conditions which can only be influenced in a limited way, this resulting in an uncontrolled deterioration of the mixing behavior in the transverse direction. Accordingly, a uniform efficient thorough mixing in the transverse direction is difficult to be carried out with conventional means and is very disturbance-sensitive.